1. Field of the Invention
The present invention relates to a diffraction grating recording medium. More particularly, the present invention relates to a diffraction grating recording medium with high recognition degree and high difficulty in forging.
2. Description of Related Art
Credit cards, cashes, checks, and other important files need to have a feature of being not easy to be duplicated. However, the recent duplicating equipment and duplicating technique become increasingly developed, so it is also necessary to develop an updated anti-forgery technique. Hologram is a diffractive optical element, and is widely applied in authentication, security, anti-forgery, and other purposes, such as holographic pigeon pattern or holographic globe pattern on the credit cards. The main reason is that it is impossible to forge the hologram by means of scanning or printing, and it is not easy to forge in a holographic manner except for the experts familiar with the holographic technique. The diffraction grating recording medium has an advantage of being suitable for mass production, so it is usually applied in holographic anti-forgery technique.
FIG. 1 is a schematic view of a conventional diffraction grating recording medium. Referring to FIG. 1, a diffraction grating recording medium 100 has a substrate 110, and the substrate 110 is divided into a plurality of regions 120 including a region 120a to a region 120e. Each region 120 has an adhesion layer 130 and a plurality of diffractive elements 140, and the adhesion layer 130 fixes the diffractive elements 140 on the substrate 110. The diffractive elements 140 in each region 120 are arranged with different patterns, so that the regions 120 can display different colors and generate a holographic image.
FIG. 2A is a schematic view of diffraction gratings of different colors of another conventional diffraction grating recording medium, and FIG. 2B is a schematic view of the disposition of diffraction gratings in FIG. 2A. Referring to FIGS. 2A and 2B, diffraction gratings 210 in FIG. 2A include red diffraction gratings R0-R255, green diffraction gratings G0-G255, and blue diffraction gratings B0-B255, capable of generating primary color with different intensities, respectively Different diffraction gratings 210 are chosen to provide different colors. A holographic pixel 200 in FIG. 2B includes a plurality of mixed diffraction gratings 210, so as to provide required colors.
In the holographic pixel 200, the diffraction gratings 210 are conventional straight line gratings, such that the colors can only be adjusted by adjusting pitch of grating, so it is difficult to increase viewing angle of the holographic pixel 200.